Automatically adjust contig size cutoff and beta

[jakobnissen]

It would be nice if Vamb could automatically adjust the contig size cutoff, based on the characteristics on the input data. To do this, we need to:

• [x] Determine the optimum size cutoff for short and long read data
• [x] Determine if the presence of smaller contigs interfere with mapping
• [x] Determine how few contigs it takes for Vamb to overfit

[jakobnissen]

Investigated by @Las02

• Sequence cutoffs between 500-2000 bp matters little. For plasmids, 200 gets the best results, but the effect is small and unclear.
• Smaller contigs does not interfere with mapping. There is no difference between filtering contigs before mapping, and in Vamb.
• We were unable to get Vamb to show any signs of overfitting even when using 10% of the contigs of a pretty good assembly. It's still possible it can overfit if we have truly few, huge contigs, but we cannot determine this until we get good synthetic long-read datasets

So, in conclusion: None of this matters. We can remove the overfitting warning from Vamb